Power amplifiers of class A or B have been used as the linear transmitter for radio frequency. But such power amplifier is defective in that the power efficiency remarkably deteriorates in the low level region when it is used to amplify modulated signals which changes drastically in the level of the envelope. In RF transmitters of portable type which use batteries as the power source, for example, such problems arise as duration of transmittable time becomes short or a large and heavy battery becomes necessary.
In order to overcome such problems, the present applicant has filed a patent application for the linear power amplifier of high efficiency (Japanese Patent Application laid-open Sho 62-274906). FIG. 1 shows the construction of an embodiment of the linear transmitter using above-mentioned linear power amplifier.
An input terminal 1 is inputted with signals to be modulated in analog or digital form. A modulator 2 generates linear modulated signals based on the above signals. A high-frequency saturated power amplifier 4 amplifies the linear modulated signals and outputs the same at an output terminal 9.
The power amplifier 4 includes an input matching circuit 41, a FET 42 as an amplification device, an RF choke coil 43 and an output matching circuit 44, and makes the saturated output level of the amplifier 4 correspond to the envelope of the input signals by controlling the drain bias voltage V.sub.D of the FET to be substantially proportional to the envelope of the input signals. This type of control enables the amplifier 4 to operate as a linear amplifier while maintaining it at saturation condition with a high efficiency to remarkably reduce the distortion in outputs.
The drain bias voltage V.sub.D may be obtained by controlling the power voltage inputted at a power supply terminal 8 by a DC voltage controller 7. The DC voltage controller 7 includes a DC-DC converter or a series-pass regulated transistor, and is controlled by drain voltage control signals V.sub.C. The control signals V.sub.C are obtained by dividing with a coupler 3 the linear modulated signals fed from the modulation 2 to the amplifier 4, detecting the envelope thereof with an envelope detector 5 and compensating the detector output V.sub.C ' using a compensating circuitry 6.
As the above-mentioned prior art uses a saturation type radio frequency power amplifier of high efficiency, theoretically it can conduct linear amplification at a remarkably high efficiency. For instance, if a saturated power amplifier of power efficiency of 70% is used as the amplifier 4 and a DC-DC converter of 75% efficient is used as the circuit 7, the linear amplification may be conducted at combined efficiency of 50% or higher.
FIG. 2 is a block diagram to show an embodiment of the compensate circuitry 6.
An envelope detector 5 includes a RF diode or other parts of non-linear analog circuit, and characteristics of detector thereof change due to changes in temperature. The current temperature is detected by a temperature sensor 64, and the detector output V.sub.C of the detector 5 is compensated based on the temperature by an operational circuitry 62.
The detector output V.sub.C ' is digitized by an A/D converter 61 and inputted at an operational circuitry 62. The circuitry 62 reads a compensating value for the diode characteristic which corresponds to the output of the temperature sensor from a memory 65 which stores diode characteristics and compensates the input signals with the compensated values. The result is converted into analog signals by D/A converter 63 and outputted.
However, the prior art linear transmitter fluctuates in its detector characteristics because of temperature changes as well as manufacture errors as the envelope detector comprises RF diode or other parts of non-linear analog circuit. Therefore, the drain control signals are extremely difficult to improve their precision, and errors in drain bias make it difficult to reduce distortion satisfactorily.
This problem may be solved by providing a compensate circuitry, but the construction of such the circuitry inevitably becomes more complicated and expensive.
This invention aims to overcome those problems encountered in the prior art and aims to provide a linear transmitter which can conduct linear power amplification at high efficiency with less distortion but without the necessity to use complicated compensate circuitries.